1. Field of the Invention
This invention relates to rubber tires for use in subways, monorails, electric vehicles for a new transportation system, airplanes, automobiles, construction vehicles, industrial vehicles and the like. More particularly it relates to a rubber tire having good flame retardance and safeness while maintaining various rubber properties such as wear resistance, resistance to heat separation, crack resistance and the like.
2. Description of Related Art
In general, rubber tires have a property of easily burning when the tire is exposed to a high-temperature flame or sparked by short-circuit at high voltage and current. Further, there is a risk that once the tire is ignited and rendered into a burning state, the fire cannot easily be extinguished but also that spreading is promoted due to high burning temperature. Further poisonous gas containing carbon is produced.
In case of the electric vehicle provided with rubber tires for transporting a great number of persons, the burning of these tires endangers human life. Particularly, it becomes more serious in case of the subway vehicles because these vehicles are run in a closed space.
In case of an emergency state where it is particularly obliged to apply severer braking force to the rubber tires in the landing of the airplane, there is a risk that the tread portion locally raises temperature and finally ignites. On the other hand, when flammable and explosive hazardous materials are transported by automobiles, there may be caused a risk that the temperature of the rubber tire becomes abnormally high and finally the ignition is caused. Therefore, it is strongly demanded to develop non-ignitable and flame-retardant rubber tires.
In this connection, Japanese patent laid open No. 1-273703 proposes a flame-retardant rubber tire in which a flame-retardant rubber layer of 0.3-3.0 mm in thickness is formed on a rubber surface of at least one sidewall portion. This tire is particularly used in a monorail or the like, wherein the flame-retardant rubber layer formed in the sidewall portion of the tire is located in the vicinity of contact brushes of an electric motor generating electric spark.
In the discussion on the burning of the rubber tire used in the electric vehicle, it should be noted that an electric current is grounded onto a railway through a tread surface of the tire contacting therewith in the leakage of electricity. Since a large current is generally used in the vehicle, a violent sparking phenomenon is created between the tread and the railway together with an explosion sound. Even when such a sparking occurs over a very short time, there is a risk that the tread of the rubber tire is ignited to cause the firing or burning of the tire. In order to avoid such a risk, it has been attempted to extremely increase the electric resistance of the rubber tire to, for example, several to several tens M.OMEGA. for a while, but some problems such as risk of electric shock to human body, bad influence upon other parts and the like are posed. Therefore, it is considered that the existing rubber tires having a low electric resistance are suitable. Moreover, these facts are not considered at all in the aforementioned conventional technique that the flame-retardant rubber layer is formed only in the sidewall portion of the rubber tire.
Further, it is clear that the above problems are existent even in the rubber tires for airplane and automobiles. In the rubber tire for automobiles, it is considered that the tread may ignite by some causes, for example, a cause that separation failure is caused in the belt of plural steel cord layers for the reinforcement of the tread to rub the steel cords with each other to thereby raise the temperature and form cracks in the tread. Therefore, the conventional technique for the formation of the flame-retardant rubber layer in the sidewall portion is incomplete and impractical against not only the ignition from the inside of the tire but also the exposure to high-temperature flame from outside.
According to the inventors' experiments, it has been confirmed that even when flame is locally applied to the sidewall portion of the flame-retardant rubber layer, the rubber tire easily ignites and burns according to the weight and gauge of the flame-retardant rubber layer dependent upon heat capacity of fire source. In this connection, the above conventional technique has a problem in practical use.
Since the wear resistance, resistance to heat generation, separation resistance and the like are preferentially considered as a most fundamental and important property of the rubber tire and there is a fear of largely damaging these properties with the use of the flame-retardant rubber composition in the tread, it is considered that there is no technique for the formation of flame-retardant rubber tires capable of coping with various environmental conditions promoting such burning.